Tracking geomorphic and vegetative recovery: implications for flow hydrology and river management

Throughout Eastern Australia, and much of the World, river systems have undergone a series of hydro-geomorphic and vegetative changes following European settlement. Recent research has shown that river recovery is occurring, through improved geomorphic and vegetative condition, but there is limited evidence on the composition and effect of increased riparian vegetation on roughness and flood hydrology. Using the Allyn River, New South Wales, Australia as a case study, historical aerial photographs and satellite imagery showed that significant geomorphic recovery has occurred since the 1970s and total riparian vegetation canopy cover has increased from 30.4% in 1940 to 38% in 1967 and 62.9% in 2016. Riparian vegetation roughness, calculated using a terrestrial laser scanner gap fraction method and retrospective analysis, showed an increase in average Manning’s n from 0.0156 in 1940 to 0.0194 in 1967 to 0.0372 in 2016 (2 m stage height). As a result, flood wave travel time has attenuated by over 8 hours for ~3 m flows, but further river recovery is required to attenuate high flows (>4 m). This study highlights the outcomes of catchment-scale passive river recovery and provides improved methods for quantifying site- and stage-height- specific vegetative roughness.